1. Field of the Invention
The present invention relates generally to enlarging the diameter of a subterranean borehole and, more specifically, to enlarging the borehole below a portion thereof which remains at a lesser diameter. The reaming method and apparatus of the present invention includes the capability to drill out cement and float equipment resident in a casing above the borehole interval to be enlarged with substantially no damage to the casing interior or the reaming apparatus. The reaming method and apparatus of the present invention also includes the capability to clean up and remove cement, cement float equipment, debris, and other contaminates that have formed restrictions within a cased or open borehole.
2. State of the Art
It is known to employ both eccentric and bi-center bits to enlarge a borehole below a tight or undersized portion thereof.
An eccentric bit includes an eccentrically, laterally extended or enlarged cutting portion which, when the bit is rotated about its axis, produces an enlarged borehole. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738.
A bi-center bit assembly employs two longitudinally superimposed bit sections with laterally offset axes. The first axis is the center of the pass-through diameter, that is, the diameter of the smallest borehole the bit will pass through. This axis may be referred to as the pass-through axis. The second axis is the axis of the hole cut as the bit is rotated. This axis may be referred to as the drilling axis. There is usually a first, lower and smaller diameter pilot section employed to commence the drilling, and rotation of the bit is centered about the drilling axis as the second, upper and larger diameter main bit section engages the formation to enlarge the borehole, the rotational axis of the bit assembly rapidly transitioning from the pass-through axis to the drilling axis when the full-diameter, enlarged borehole is drilled.
Rather than employing a one-piece drilling structure such as an eccentric bit or a bi-center bit to enlarge a borehole below a constricted or reduced-diameter segment, it is also known to employ an extended bottomhole assembly (extended bi-center assembly) with a pilot bit at the distal or leading end thereof and a reamer assembly some distance above. This arrangement permits the use of any bit type, be it a rock (tri-cone) bit or a drag bit, as the pilot bit. Further, the extended nature of the assembly permits greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will take the path intended for the borehole. This aspect of an extended bottomhole assembly is particularly significant in directional drilling.
While all of the foregoing alternative approaches can be employed to enlarge a borehole below a reduced-diameter segment, the pilot bit with reamer assembly has proven to be highly effective. The assignee of the present invention has, to this end, designed as reaming structures so-called xe2x80x9creamer wingsxe2x80x9d in the very recent past, which reamer wings generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof and a tong die surface at the bottom thereof, also with a threaded connection. As an aside, short-bodied tools frequently will not include fishing necks, including the short-bodied reamer wings designed by the assignee of the present invention. The upper midportion of the reamer wing includes one or more longitudinally extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive (also termed xe2x80x9csuperhardxe2x80x9d) cutting elements; commonly such superabrasive cutting elements, or cutters, are frequently comprised of PDC ( Polycrystalline Diamond Compact) cutters. The lower midportion of the reamer wing may include a stabilizing pad having an arcuate exterior surface of the same or slightly smaller radius than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizer pad is characteristically placed on the opposite side of the body with respect to the reamer wing blades so that the reamer wing will ride on the pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut.
While the aforementioned reamer wing design enjoyed some initial success, it was recognized that the device as constructed might not effectively and efficiently address the problem or task of achieving a rapid transition from pass-through to full-hole or xe2x80x9cdrillxe2x80x9d diameter which closely tracks the path of the pilot bit and which does not unduly load the blades or bottomhole assembly during the transition. Since a reamer wing may have to re-establish a full diameter borehole multiple times during its drilling life in a single borehole, due to washouts and doglegs of the pilot hole, a rapid transitioning ability when reaming is restarted as well as a robust design which can accommodate multiple transitions without significant damage was recognized as a desirable characteristic and design modification. U.S. Pat. No. 5,497,842, assigned to the assignee of the present invention and hereby incorporated by reference herein, discloses the use of so-called xe2x80x9csecondaryxe2x80x9d blades on the reamer wing to speed the transition from pass-through to drill diameter with reduced vibration and borehole eccentricity.
While the improvement of the ""842 patent has proven significant, it was recognized that further improvements in the overall stability of the bottomhole assembly, including transitioning from pass-through to drill diameter, would be highly desirable. One problem the prior art reamer assembly designs have experienced is undue vibration and even so-called bit xe2x80x9cwhirl,xe2x80x9d despite the focused or directed force vector acting on the reaming assembly and the presence of the stabilization pad. These undesirable phenomena appear to be related to the configuration of the stabilization pad (illustrated in FIG. 5 of the ""842 patent), which engages the borehole wall axially and circumferentially under the radially directed resultant force vector of the reamer wing as the assembly drills ahead in the pilot hole, due to the pad""s abrupt radial projection from the reamer wing body. Furthermore, it was observed that the entire bottomhole reaming assembly as employed in the prior art for straight-hole drilling with a rotary table or top drive often experiences pipe xe2x80x9cwhipxe2x80x9d due to lack of sufficient lateral or radial stabilization above the reamer wing. In addition, reaming assemblies driven by downhole steerable motors for so-called directional or navigational drilling experienced problems with stability under the lateral forces generated by the reamer wing so as to make it difficult to maintain the planned borehole trajectory.
U.S. Pat. No. 5,765,653, assigned to the assignee of the present invention and hereby incorporated by reference herein, addresses the aforementioned problems by providing an axially as well as circumferentially tapered pilot stabilizer pad (see FIGS. 4, 6, 7 and 7A of the ""653 patent), to which may optionally be added one or more eccentric stabilizing elements above the reaming apparatus (see FIGS. 8-12 of the ""653 patent).
One remaining problem with the use of state of the art reaming apparatus is the inability to rotate the apparatus while passing the reaming apparatus through a casing above a borehole interval to be enlarged without damage to the casing interior or to the apparatus. This is due, in large part, to the fact that there are typically, but not necessarily, three points of contact (also termed xe2x80x9cpass-through pointsxe2x80x9d) between the casing and the reaming apparatus, a stabilization pad as disclosed in the aforementioned ""842 and ""653 patents, and radially outer edges of two of the blades of the apparatus. It is the two outer blade edges which are of primary concern, as PDC cutters thereon may scrape and damage the casing interior, and damage to the PDC cutters from such contact may shorten the life of the apparatus and even cause it to drill an under-gage enlarged borehole below the casing. Further, the inability to rotate the reaming apparatus without such damage effectively precludes rotation of the apparatus to remove float equipment still present within the borehole that was used in cementing the casing into the borehole.
Additionally, not being able to rotate the reaming apparatus without likely incurring such damage is a major impediment in clearing a column of cement residing in the casing above the float equipment or in clearing a portion of casing that has become constricted with scale, chalk, mineral deposits, sand, paraffin, wax or other deposits or debris. Since rotation within the interior of a casing will, of necessity, be around the pass-through axis of the apparatus rather than about the drill axis of the apparatus, the pilot bit is thus rotated eccentrically about the casing interior, so that contact with cement and float equipment, for example, within the casing causes substantial lateral forces which impel the reamer blades against the casing wall, to the detriment of both the blades and the casing. Thus, it is typically necessary to drill out the casing and float equipment with another bit or milling tool, separately trip that bit or tool out of the borehole, and subsequently run the reaming apparatus into the borehole, an obviously time-consuming and expensive process. Therefore, there remains a need for a reaming apparatus capable of casing drill out without substantial damage to the casing interior or to the apparatus itself before continuing on to enlarging the borehole below the constriction provided by the casing or other mission.
A further need within the art is for a reaming apparatus capable of being used for both casing drill out as well as open, or uncased, borehole drill-out of cement, cement float equipment, debris, and borehole contaminants such as scale, wax, paraffin, or other unwanted substances which typically adhere to and form a buildup on the interior of the casing or borehole, especially when the borehole is being used to produce hydrocarbons.
The present invention provides a design for an apparatus for enlarging the diameter of a borehole, which reaming apparatus may also be termed a ream while drilling (RWD) tool or a xe2x80x9creamer wingxe2x80x9d. The reaming apparatus of the invention is effective in enlarging the borehole diameter and also affords protection for the structure of the apparatus while running through casing, as well as for casing through which the tool is run. Thus, drill out of cement and float equipment resident in the casing may be effected without damage to the casing interior or to the reaming apparatus so that, after drill out, the tool may effect the desired enlargement of the borehole below the casing. While it is contemplated that the reaming apparatus of the invention will commonly be run above a pilot bit for drilling the borehole to be enlarged immediately thereafter by the reaming apparatus, the invention is not so limited, as it may be applied to enlarging an existing borehole.
One aspect of the invention comprises longitudinally and circumferentially enlarging the surface area of radially outer surfaces of gage pads on blades of the tool above the superabrasive PDC cutters (as the tool is normally oriented during drilling) which contact the casing interior during rotation therein, and providing such surfaces with bearing elements exhibiting bearing surfaces to ride on the casing interior. The bearing surfaces may preferably be overexposed (e.g., extend radially beyond) with respect to the drill diameter of the tool to provide additional protection while passing through the casing, and be formed of a material which will quickly wear after passage through the casing when reaming therebelow. Further, the bearing surfaces may be enhanced in terms of size, orientation and conformity to the interior wall of the casing by providing at least one flat thereon. Moreover, the bearing elements may optionally comprise superabrasive cutting elements, such as PDC cutting elements having suitably configured tables having surfaces oriented so as to be generally nonaggressive and thus allowing selected surfaces of the superabrasive table of the superabrasive elements to serve as bearing surfaces.
It is also contemplated that one or more ovoid, or at least partially hemispherical, headed tungsten carbide compacts may be placed on the radially outer surface of a blade and facing generally radially outwardly, for example, on a rotationally trailing blade and/or on a rotationally leading blade, thus being circumferentially offset from a given blade, to provide an additional pass-through point to accommodate the erratic rotational pattern of the tool in the casing during drill out. Such compacts may also be provided with a PDC or other superabrasive material bearing surface over at least a portion of the head.
Another aspect of the invention comprises positioning superabrasive cutters, such as PDC cutters, on a blade so as to be circumferentially and rotationally offset from a radially outer, rotationally leading edge portion of a blade where a casing contact point is to occur. Such positioning of the cutters rotationally, or circumferentially, to the rear of the casing contact point located on the radially outermost leading edge of the blade allows the cutters to remain on proper drill diameter for enlarging the borehole, but are, in effect, recessed away from the casing contact point.
Still another aspect of the invention comprises forming the rotationally leading edges of the reamer blades to be nonaggressive, which design may be employed with the pilot bit blades if a drag bit is employed for same.
Yet another aspect of the invention comprises reducing the aggressiveness of superabrasive gage cutters, such as PDC gage cutters, on the blades which are likely to contact the casing through the use of so-called carbide supported edge (CSE) PDC cutters in accordance with U.S. Pat. No. 5,460,233, such PDC cutters also preferably having a relatively large bevel, or rake land, on the diamond table in accordance with U.S. Pat. No. 5,706,906 and related U.S. Pat. No. 6,000,483. Each of the three foregoing patents is assigned to the assignee of the present invention and is hereby incorporated by reference herein.
Another feature for reducing aggressiveness of the gage is the use of one or more ovoid or bullet-shaped elements exhibiting at least partial hemispherical heads facing in the direction of rotation, preferably immediately below the gage and at least slightly leading the superabrasive gage cutters. These elements may be in the form of tungsten carbide compacts or may comprise so-called xe2x80x9cdomexe2x80x9d PDC cutters having at least partial hemispherical leading surfaces. In lieu of discrete elements, a suitably shaped protrusion may be formed on one or more blades at suitable locations, and hardfacing applied thereto by techniques known in the art.
A still further aspect of the present invention is the use of bearing elements, such as tungsten carbide (WC) inserts, diamond inserts, diamond grit-filled WC inserts, or ovoid-headed elements in the outer bearing surfaces of the pilot stabilizer pad to control wear of the pad over an extended drilling interval.
An additional aspect of the invention resides in the use of side- or rotationally backward-facing cutters on the pilot bit to assist during drill out and to protect the rotationally forward-facing cutters of the pilot bit from impact-induced delamination of the diamond tables from their substrates during the eccentric rotation of the pilot bit as drill out is effected. Omnidirectional cutters, as disclosed and claimed in U.S. Pat. No. 5,279,375, assigned to the assignee of the present invention and hereby incorporated by reference herein, may also be employed on the pilot bit to assist in drill out and for cutter protection.
A further aspect of the invention comprises, where the tool has a sufficient number of blades so that a radially outer edge of one or more blades is substantially removed from any proximity to the casing interior, placement of PDC cutters on those blades at extended radial positions to define a diameter in excess of the drill diameter to provide a safety margin in terms of ensuring a reamed interval of adequate diameter and to extend the interval of drill diameter drilled by the reamer blades by initially taking wear on the extended-radius cutters.
An additional aspect of the invention comprises a method employing the reamer apparatus of the present invention for passing through an interior diameter restriction of a casing such as a patch landing shoulder, for cleaning up and drilling out cased boreholes below such interior diameter restriction, as well as for cleaning up and drilling out open, or uncased, boreholes. Such drilling out includes, but is not limited to, reaming at least one or more portions of the borehole to an internal diameter approaching the borehole""s original internal diameter. Therefore, the subject method is particularly suitable for the cleaning out and/or drilling out of patch landing shoulders in casing, cement, cement float equipment, debris, and borehole contaminants such as scale, wax, paraffin, mineral deposits, or other unwanted substances which typically adhere to and form a buildup on the interior of the casing or borehole and is especially useful for cleaning up cased and uncased boreholes used to produce hydrocarbons.